1. Field of the Invention
The present invention relates to optical information recording and reproducing apparatuses, such as optical disc drive apparatuses, and more particularly, to a technique for detecting a tilt signal representing a tilt of a head unit, including a solid immersion lens (hereinafter abbreviated as SIL) and an objective lens.
2. Description of the Related Art
To increase a recording density of an optical disc, it is necessary to reduce a beam spot diameter on a recording surface of the optical disc. The beam spot diameter can be reduced by reducing a wavelength of light used for recording and reproducing information, and by increasing a numerical aperture (NA) of an objective lens. An objective lens having a lens element called an SIL, which is brought extremely close to the recording surface with a distance therebetween set to a fraction (for example, ½) of the recording wavelength, has been developed, to achieve an NA of one or more, even in air. Examples of such a structure are described in, for example, Japanese Journal of Applied Physics, vol. 44 (2005), pages 3564-3567, “Near Field Recording on First-Surface Write-Once Media with an NA=1.9 solid immersion lens” (hereinafter called Document 1), and in Optical Data Storage 2004, Proceedings of SPIE vol. 5380 (2004) “Near-field read-out of a 50-GB first-surface disk with NA=1.9 and a proposal for a cover-layer-incident, dual-layer near-field system” (hereinafter called Document 2).
The structure of an optical pickup included in an optical information recording and reproducing apparatus for near-field recording described in Document 1 (Japanese Journal of Applied Physics, vol. 44 (2005), pages 3564-3567) will be described with reference to FIG. 8. A light beam having a wavelength of 405 nm is emitted from a semiconductor laser 1, is collimated by a collimator lens 2, and is incident on beam shaping prisms 3, where an isotropic light-intensity distribution is obtained. Then, the light beam passes through a non-polarizing beam splitter (NBS) 4, a polarizing beam splitter (PBS) 7, and a quarter-wave plate (QWP) 8, which changes the polarization of the light beam from linear to circular. The light beam reflected by the non-polarizing beam splitter (NBS) 4 is received by a photodetector (LPC-PD) 6, which is used for controlling the emission power of the semiconductor laser 1. The light beam that passes through the quarter-wave plate (QWP) 8 is incident on an expander lens 9. The expander lens 9 corrects spherical aberrations generated by an objective lens 10 and an SIL 11, which will be described below, and includes two lenses spaced from each other by a distance that can be controlled in accordance with the spherical aberrations. The light beam from the expander lens 9 is incident on the objective lens 10 in a head unit 50. The head unit 50 includes the objective lens 10 and the SIL 11, which are mounted on actuators (not shown) that drive the two lenses together in a focusing direction, a tracking direction, and a tilt direction.
Only when the distance between the bottom surface of the SIL 11 and the surface of an optical disc 12 is a fraction of the wavelength of the light source (405 nm), for example, 100 nm or less, evanescent light that emanates from the bottom surface of the SIL 11 affects the recording surface so that information can be recorded or reproduced with a beam spot diameter corresponding to an effective numerical aperture NAeff. The above-mentioned distance is maintained by a gap servo system, which will be described below.
Referring to FIG. 8 again, a returning path of the light beam in the optical system will be described. When the light beam is reflected by the optical disc 12, the direction of circular polarization of the light beam is reversed. The reflected light beam is incident on the SIL 11 and the objective lens 10, where the light beam is collimated again. Then, the light beam passes through the expander lens 9 and the quarter-wave plate (QWP) 8, which changes the polarization of the light beam to linear, such that the direction of linear polarization is perpendicular to that of the light beam that travels toward the optical disc 12. Then, the light beam is reflected by the polarizing beam splitter (PBS) 7 and passes through a half-wave plate (HWP) 13, which rotates the plane of polarization of the light beam by 45°. An s-polarized component of the light emitted from the half-wave plate (HWP) 13 is reflected by a polarizing beam splitter (PBS) 14, passes through a lens 15, and is collected on a first photodetector (PD1) 16, so that information recorded on the optical disc 12 is reproduced as an RF output 17. A p-polarized component of the light emitted from the half-wave plate (HWP) 13 passes through the polarizing beam splitter (PBS) 14, is reflected by a mirror 18, passes through a lens 19, and is collected on a second two-division photodetector (PD2) 20. Accordingly, a tracking error 21 is output.
A portion of the light beam that corresponds to NAeff<1 and that does not cause total reflection at the bottom surface of the SIL 11 is reflected, such that the direction of circular polarization of the light beam is reversed, similar to the light reflected by the optical disc 12. A portion of the light beam that corresponds to NAeff≧1 and causes total reflection at the bottom surface of the SIL 11 is reflected, such that a phase difference δ is generated between the p-polarized component and the s-polarized component, and the polarization is changed from circular to elliptical. The phase difference δ is expressed as follows:tan(δ/2)=cos θi×√(N2×sin 2θi−1)/(N×sin 2θi)  (1)
Therefore, after the light beam passes through the quarter-wave plate (QWP) 8, the light beam includes a component polarized in the same direction as that of the light beam that travels toward the optical disc 12. This component passes through the PBS 7, is reflected by the NBS 4, passes through a lens 26, and is collected on a third photodetector (PD3) 27. The light intensity of this component is gradually reduced as the distance between the bottom surface of the SIL 11 and the optical disc 12 is reduced in the near field range, and therefore, can be used as a gap error 28. Accordingly, gap servo control can be performed by setting a target threshold in advance, and causing a gap servo circuit (not shown) to operate, such that the distance between the bottom surface of the SIL 11 and the surface of the optical disc 12 is maintained at a desired distance of 100 nm or less.
The gap servo control is described in detail in the above-mentioned Document 1 (Japanese Journal of Applied Physics, vol. 44 (2005), pages 3564-3567).
The light beam used for the gap servo circuit is not modified by the information recorded on the optical disc 12. Therefore, the gap error 28 can be reliably obtained irrespective of the presence/absence of recorded information.
FIG. 6 shows the third photodetector (PD3) 27. The third photodetector (PD3) 27 is a two-division photodetector having two sections A and B. As described above, the light intensity of the light beam that corresponds to NAeff≧1 and causes total reflection is gradually reduced as the distance between the bottom surface of the SIL 11 and the optical disc 12 is reduced in the near field range. Therefore, a tilt signal representing a relative tilt between the bottom surface of the SIL 11 and the surface of the optical disc 12 can be obtained by detecting a difference signal representing a difference in light intensity of the light returning from the bottom surface of the SIL 11 and the surface of the optical disc 12 between sections A and B. The tilt signal is input to a tilt control circuit 30 (shown in FIG. 8), which performs tilt servo control by outputting a signal to a voice coil motor (not shown) in an actuator mounted on the head unit 50, so as to prevent the bottom surface of the SIL 11 and the surface of the optical disc 12 from coming into contact with each other.
The tilt servo control is described in detail in Optical Data Storage 2006, “Cover-Layer Incident Near-Field Recording: Towards 4-Layer Discs using Dynamic Tilt Control” (hereinafter called Document 3).
As described above, the objective lens 10 and the SIL 11 are adjusted by the voice coil motor (not shown) of the actuator mounted on the head unit 50. The head unit 50 drives the actuator (not shown), such that the distance between the SIL 11 and the optical disc 12 is maintained at a predetermined distance using the gap error 28, based on a sum signal from the third two-division photodetector (PD3) 27. In addition, the tilt control circuit 30 outputs a signal for correcting the relative tilt between the bottom surface of the SIL 11 and the surface of the optical disc 12 using a tilt error 31 based on a difference signal from the third two-division photodetector (PD3) 27.
However, the optical pickup used in the known optical information recording and reproducing apparatus for near-field recording has the following problems. That is, in the known apparatus, the gap error 28 obtained by the third two-division photodetector (PD3) 27 is used to maintain the distance between the SIL 11 and the optical disc 12 at a predetermined distance, and the tilt error 31 is used to correct the relative tilt between the bottom surface of the SIL 11 and the surface of the optical disc 12. However, as shown in FIG. 7, in the process of detecting the tilt error 31 based on a difference signal between the divided sections A and B of the third photodetector (PD3) 27, there is a possibility that the head unit 50 will move in the radial direction of the optical disc 12 (X direction in FIG. 7) due to eccentricity of the optical disc 12, or the like. In such a case, the beam spot on the third photodetector (PD3) 27 also moves at the same time. Therefore, an offset is included in the tilt error 31 obtained by the third photodetector (PD3) 27, and it is difficult to accurately detect the tilt.
As described above, the distance between the end face of the SIL 11 and the surface of the optical disc 12 is 100 nm or less, and, therefore, the relative tilt between them must be controlled with high accuracy. If there is an additional error factor, such as the influence of movement of the head unit 50, as described above, the accuracy of the tilt servo control is degraded.